Collagen-induced arthritis (CIA) in rats is a well-established experimental animal model that has been used to gain insight into the pathogenesis of rheumatoid arthritis (RA), a chronic and commonly disabling disease with a prevalence of 1% in most populations. Both CIA and RA are complex trait diseases regulated by MHC and non-MHC genes, and both have an increased susceptibility among females, with a RA female to male ratio of 3:1. The factors that account for the increase female preponderance of RA are poorly understood. Gonadal hormones, particularly a reduction in their levels, have been implicated in disease pathogenesis, but that does not account for all the gender differences in disease expression. The underlying hypotheses of this proposal are that the non-MHC loci governing susceptibility and severity of CIA in rats will be highly relevant to the pathogenesis of RA, and also that at least part of the genetic susceptibility to CIA differs between females and males, with certain genes regulating disease in one gender but not in the other. The identification of these sex-specific genes, and the sex factors regulating their penetrance will be of great relevance to the understanding of the factors accounting for the increased female susceptibility, and could generate new targets for the development of more specific therapies, for prevention, diagnosis and prognosis. In a study of CIA in a F2 intercross between MHC identical inbred CIA-susceptible DA rats and CIA-resistant ACI rats, gender had a major effect in susceptibility and in arthritis severity. A genome-wide scan done separately for females and males led to the identification of a new non-MHC locus, Cia12, on chromosome 12, that regulates CIA in females, accounting for 24% of the genetic contribution to the variance in arthritis severity, but not in males. The rat Cia12 interval, and its syntenic regions in the mouse and human genomes contain loci regulating several forms of autoimmune diseases, including arthritis, suggesting that this gene may be relevant to different diseases, underscoring the broader relevance of this study. To identify this gene and to confirm and characterize its sex-specific effect a combined approach was planned. On Aim 1, speed-congenic lines will be generated, introducing the Cia12 interval from ACI rats into DA background, and vice-versa, and both genders will be studied separately for CIA. On Aim 2, gonadal hormones and neuro-endocrine manipulation are anticipated to determine the sex-specific mechanisms regulating Cia12. On Aim 3, sub- phenotypes involved in the autoimmune response, perhaps more directly regulated by Cia12 than the complex arthritic phenotype, will be identified and used in the narrowing of the interval of interest. This aim will also provide additional insight into the Cia12 gene function. On Aim 4 the specific gene will be identified based on candidate gene analysis or positional cloning, and disease-causing allelic variations characterized.